Chance discovery could lead to better cheaper batteries

By Jack Loughran

Published Tuesday, April 19, 2016

A rechargeable battery with much greater energy density than conventional car batteries, but at the same cost has been developed by a team at the US Department of Energy's Pacific Northwest National Laboratory.

The new battery could also become a cost-effective, environmentally friendly alternative for storing renewable energy and supporting the power grid.

Its developers trialled a zinc-manganese oxide battery as an alternative to the lithium-ion technology found in modern devices but discovered that it operated in way they had not anticipated.

Lithium-ion batteries store and release energy through a process called intercalation, which involves lithium ions entering and exiting microscopic spaces in between the atoms of a battery's two electrodes.

But the team found their new device was undergoing an entirely different process. Instead of simply moving the zinc ions around, their zinc-manganese oxide battery was undergoing a reversible chemical reaction that converted its active materials into entirely new ones.

Zinc-manganese technology is theoretically very attractive because it is as inexpensive as lead-acid batteries due to the low cost of the materials used but has greater energy density.

However, earlier attempts to use these materials resulted in batteries that were unable to absorb any more energy after just a few charging cycles.

The team went back to the drawing board and built their own battery with a negative zinc electrode, a positive manganese dioxide electrode and a water-based electrolyte in between the two.

They tested the small, button-sized batteries, repeatedly charging and discharging them, and found they were quickly unable to store energy, just like in previous attempts.

"The idea of a rechargeable zinc-manganese battery isn't new; researchers have been studying them as an inexpensive, safe alternative to lithium-ion batteries since the late 1990s," said Jun Liu, who worked on the project.

"But these batteries usually stop working after just a few charges. Our research suggests these failures could have occurred because we failed to control chemical equilibrium in rechargeable zinc-manganese energy storage systems."

Zinc-manganese batteries lose their storage capacity because manganese from the battery's positive electrode begins to degrade, making the battery's active material inaccessible for energy storage.

To avoid this, the team slowed down this degradation significantly by increasing the electrolyte's initial manganese concentration.

They added manganese ions to the electrolyte in a new test battery which was then able to reach storage capacity of 285mAh per gram of manganese oxide over 5,000 cycles, while retaining 92 per cent of its initial storage capacity.